Cheesy Onion Mashed Potato Casserole (Printable)

Creamy mashed potatoes with caramelized onions and melted cheese, baked until golden and bubbly.

# What You Need:

→ Potatoes

01 - 2 lbs russet or Yukon Gold potatoes, peeled and cubed

→ Dairy

02 - 1 cup whole milk
03 - 4 tbsp unsalted butter, plus extra for greasing
04 - 1 ½ cups shredded sharp cheddar cheese
05 - ½ cup shredded mozzarella cheese
06 - ½ cup sour cream

→ Vegetables

07 - 2 large yellow onions, thinly sliced
08 - 2 tbsp olive oil
09 - 2 cloves garlic, minced
10 - 2 tbsp chopped fresh chives (optional, for garnish)

→ Spices & Seasonings

11 - 1 tsp salt, plus more to taste
12 - ½ tsp freshly ground black pepper

# How To Make It:

01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with butter.
02 - Place peeled and cubed potatoes in a large pot of salted water. Bring to a boil, then reduce heat and simmer until fork-tender, about 15 to 18 minutes. Drain thoroughly.
03 - While the potatoes cook, heat olive oil in a large skillet over medium heat. Add thinly sliced onions and sauté, stirring occasionally, until golden brown and deeply caramelized, about 15 to 20 minutes. Add minced garlic and cook for 1 additional minute.
04 - Mash the drained potatoes together with butter, warm milk, sour cream, salt, and pepper until smooth and creamy.
05 - Stir half of the cheddar and mozzarella cheeses into the mashed potatoes, then gently fold in the caramelized onions. Reserve a few tablespoons of onions for the topping if desired.
06 - Spread the potato mixture evenly in the prepared baking dish. Top with the remaining cheddar and mozzarella cheeses and any reserved caramelized onions.
07 - Bake for 25 minutes, or until the cheese is melted and bubbly with lightly golden spots on top.
08 - Garnish with fresh chives if desired. Serve hot.

# Expert Tips:

01 -
  • Caramelized onions turn ordinary mashed potatoes into something that tastes like it took all day, even though your actual effort is refreshingly lazy.
  • The cheese pulls on top create that stretchy, golden moment everyone fights over before the dish even hits the table.
02 -
  • Adding cold milk or sour cream to hot potatoes will make them seize up into a gummy texture, so always warm your dairy first.
  • The casserole can be assembled up to 24 hours ahead and stored covered in the fridge, then baked straight from the refrigerator with about 10 extra minutes in the oven.
03 -
  • Drain the potatoes thoroughly and return them to the warm pot for a minute to evaporate excess moisture, because wet potatoes make a watery casserole every single time.
  • Shred your own cheese from a block instead of using pre shredded bags, as the anti caking coatings on bagged cheese prevent it from melting smoothly.

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